It is well known that many articles have been made by powder metallurgical processes comprising pressing metallic powders together at room temperture with sufficient pressure to obtain cohesion of the particles and thereby provide a compact having sufficient strength to retain its shape when the pressure is released. Frequently the compact is sintered after pressing in order to obtain improved density, uniformity and strength in the compact. In some instances it is desirable to use sintered compacts as metal stock for making wrought products. It is also known that powder metallurgical processes frequently result in a number of economic advantages such as the elimination of the need for melting and casting metals into ingots and working ingots down to sizes near those required for making the finished articles. It is also understood that making articles from metallic powders instead of from melted alloys sometimes enables production of articles from metallic compositions containing ingredients that are difficult or impossible to combine by known melting practices, e.g., compositions comprising metal oxide powders and powders of elemental metals.
In spite of the known advantages offered by the potentials of powder metallurgy, presently known processes for compacting metal powders are not wholly satisfactory for making all the shapes and sizes in which articles are desired. Production of complex shapes, such as tubing and other tubular articles, from metallic powders presents problems involving compacting powder uniformly to obtain satisfactory uniform and high density packing in the compact. Uniform and high density of compaction are generally required where high strength is needed in the finished article. Moreover, even when a certain amount of porosity is desired in the finished article, e.g., in filters or bearings, uniformity of compaction is highly desirable in order to control the porosity. Control of compaction, of course, usually involves control over application and distribution of compacting pressure. In the powder metallurgical art it is understood that as a practical matter a mass of metallic powder under pressure in a mold (or die) does not behave like a true fluid in that the applied pressure is not transmitted uniformly throughout the mass. Failure of loose powder to behave like an ideal fluid is generally attributed to friction at the mold wall and to internal friction that occurs between particles of the powder. Although such friction and pressure distribution difficulties may be somewhat alleviated by use of lubricants, compaction of metal powders by usual methods of single-ended or double-ended pressing in a mold is generally unsatisfactory for producing uniform high-density compacts when the configuration of the compact is such that the length of the compact along the direction in which it is pressed in greater than about five times the minimum cross-sectional dimension of the compact.
Heretofore, short tubular compacts, e.g., hollow cylindrical compacts having length-to-wall thickness ratios (L:T ratios) up to about 1:1, have been made by confining metal powder in an annulus between a hollow cylindrical mold and a concentrically disposed cylindrical core and then compressing the powder from one or both ends of the annulus. However, when attempting to make longer and thinner tubes it is generally found that pressing tubular compacts endwise in such a manner fails to produce uniform compaction if the L:T ratio of the tube form is greater than about 5:1. Moreover, for producing cylindrical tubular compacts having the uniformly high density that is needed in compacts which are to be sintered and cold-drawn to tubing, such single-ended and double-ended pressing methods are wholly unsatisfactory for making long, relatively thin, tubular compacts with high L:T ratios of 5:1 and higher.
Another method of fabricating tubes from powder utilizes a can. The powder is introduced into the can whereupon it is sealed. The can is placed in an extruder having a central mandrel. The can is forced against the mandrel resulting in a tube having an outer skin made from the simultaneously extruded can. The tube must be then decanned; an involved process.
It has also been proposed to produce tubular compacts by isostatic pressing methods whereby a powder annulus is enclosed in a flexible envelope and pressure from a fluid is applied simultaneously at all points around the envelope inside and/or outside of the powder. The isostatic pressing fluid may be oil, water or gas. Although isostatic pressing in flexible envelopes may produce uniform compression of powders, such processes generally suffer from a number of disadvantages including difficulties in removing entrapped air and also difficulties in obtaining close dimensional tolerances due to flexure of the envelope during filling and pressing. Further, where it is required to produce long tubes of many feet in length, the need for obtaining sufficient force to provide the required pressure simultaneously over large areas of the tube gives rise to needs for undesirably large and expensive apparatus, especially when a high production rate is required. Moreover, the requirement for maintaining an imprevious enveloping barrier between the powder and the pressing fluid presents a source of accidental failure in production inasmuch as even a small leak in the envelope can result in entrance of the fluid into the powder mass, thus spoiling the product.
Other known methods for compacting metallic powder include powder-rolling processes and stepwise intermittent compaction processes. Of course, these methods are obviously very difficult or are wholly impractical to apply to production of hollow, cylindrical articles. Moreover, intermittent stepwise compaction may lead to detrimental lack of uniformity in the product.
Although many attempts have been made to overcome the foregoing difficulties and disadvantages and other difficulties, none, so far as we are aware, have been entirely successful when carried into practice commercially on an industrial scale.
There has now been discovered a new process whereby metallic powders are compacted to provide tubular-shaped compacts having good uniformity of compaction with close control of dimensional requirements. Further, a new apparatus having special advantages for pressing metallic powders to make long tubular compacts has also been invented.